Such position-measuring devices are used, in particular, in machine tools for measuring the position of a tool relative to a workpiece to be machined, in coordinate measuring machines for determining the position and dimensions of test objects, as well as in the semiconductor industry, for example, in wafer steppers. Such a position measuring device may be an angle- or length-measuring device, in which the scale is directly attached to the drive unit (rotary motor or linear motor), or the scale is attached to a component driven by the drive unit. The scanning unit of the position-measuring device is stationarily disposed on another machine part opposite the moving scale.
The scale of such a position-measuring device is known, for example, from DE 101 32 521 A1 and has a periodic measuring graduation for generating periodic measurement signals in the form of incremental signals, as well as a reference mark for generating a reference signal. An absolute reference of the incremental position measurement can be established for the position of the reference mark via the reference signal by setting a counter to a predetermined counter value.
At the beginning of a position measurement, as well as for correcting and monitoring the counter value, it is often required to approach the reference mark starting from an arbitrary position. To this end, area markings are provided on the scale, the area markings making it is possible to distinguish where the scanning unit is located relative to the reference mark; i.e., whether the scanning unit is located on one or the other side of the reference mark. In the device known from DE 101 32 521 A1, this area marking is configured as a reflective or transparent strip on one side of the reference mark. On the other side of the reference mark, the area marking is embodied as a diffraction grating. A plurality of photoreceivers are arranged in the scanning unit for scanning both area markings. Some of the photoreceivers are only impinged upon by the scanning beam when the scanning unit is located on one side of the reference mark. In contrast, the remaining photoreceivers are only impinged upon by the scanning beam when the scanning unit is located on the other side of the reference mark. An area signal is generated from the scanning signals of the various photoreceivers. The side of the reference mark on which the scanning unit is currently located during position measurement can be inferred from the level of this signal.
The procedure for generating area signals described in DE 101 32 521 A1 is suitable for position-measuring devices with collimated illumination of the scale. However, this concept cannot be used for a position-measuring device with divergent illumination of the scale.